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Abstract

Background. We previously showed that endothelin-1 (ET-1)-induced positive inotropic response in normal hearts was reversed in heart failure (HF). However, its cellular mechanism is unclear. Since L-type calcium current (ICa,L) plays a critical role in cardiac contractility, we tested the hypothesis that altered ICa,L response to ET-1 stimulation may contribute to this ET-1-induced contrasting inotropic action.

Methods. We assessed the effect of ET-1 on ICa,L response in isolated left ventricular (LV) myocytes obtained from 16 rats with isoproterenol (ISO)-induced HF (4 months after 340 mg/kg, sq, for 2 days) and from 14 age-matched normal control rats. To examine the mechanism, ET-1 functional responses were also evaluated in 3 subgroups after the myocytes were pretreated with the ETA receptor blocker, BQ123 (10−5 mol/L), pertussis toxin (PTX, 2μg/ml, 36°C, 6 hours), and dibutyryl-cAMP (Db-cAMP, 10−4 M). ICa,L was measured using the whole-cell voltage clamp technique.

Results. In normal myocytes, compared with baseline, superfusion of 5 incremental dosages of ET-1 (10−10 to 10−6 M) significantly increased ICa, L by 4.9, 8.7, 18.4, 26.4, and 41.0%, respectively. The ICa,L augmentation was dose-dependent with EC50 8.4 nM. In HF myocytes, the baseline ICa,L was significantly lower (5.6± 0.2 vs 8.3± 0.2 pA/pF, p<0.01). The incremental dosages of ET-1 (10−10 to 10−6 M) significantly decreased ICa,L by 6.2, 9.7, 22.0, 32.2, and 39.7%, respectively. The dose-dependent inhibition of ICa,L was IC50 15.0 nM. In HF, ET-1-induced inhibition was not due to altered voltage of channel activation. These ET-1-induced changes in ICa, L were abolished by the incubation of myocytes with BQ123 in both normal and HF groups of rats. In the normal myocytes, ET-1-induced increases in ICa,L persisted with pretreatment of myocytes with PTX. However, in the HF myocytes, ET-1-induced decrease of ICa, L was prevented with pretreatment of PTX or Db-cAMP.

Conclusions. HF alters the response of ICa,L to ET-1. In normal myocytes, ET-1 enhances ICa,L. However, in HF myocytes, ET-1 depresses ICa,L. These effects are coupled with ETA receptors. The ET-1-induced inhibition of ICa,L in HF is likely to be mediated through cAMP-dependent mechanism and coupled with PTX-sensitive G-protein.